Spinal Arthroplasty Device and Method

ABSTRACT

An artificial spinal joint and methods for replacing at least a portion of a natural intervertebral disc are provided. In some embodiments, at least portions of facet joints are replaced along with the intervertebral disc.

CROSS-REFERENCE

This application claims priority from U.S. Provisional Patent Application Ser. No. 60/534,960 filed on Jan. 9, 2004, entitled “Posterior Lumbar Arthroplasty.” The following applications also claim priority to the above referenced provisional application and are related to the present application. They are incorporated by reference herein.

-   U.S. Utility patent application Ser. No. (Attorney Docket No.     P21769), filed on Jan. 7, 2005 and entitled “Dual Articulating     Spinal Device and Method;” -   U.S. Utility patent application Ser. No. (Attorney Docket No.     P21756), filed on Jan. 7, 2005 and entitled “Split Spinal Device and     Method;” -   U.S. Utility patent application Ser. No. (Attorney Docket No.     P21752), filed on Jan. 7, 2005 and entitled “Interconnected Spinal     Device and Method;” -   U.S. Utility patent application Ser. No. (Attorney Docket No.     P21745), filed on Jan. 7, 2005 and entitled “Mobile Bearing Spinal     Device and Method;” -   U.S. Utility patent application Ser. No. (Attorney Docket No.     P21743), filed on Jan. 7, 2005 and entitled “Support Structure     Device and Method;” -   U.S. Utility patent application Ser. No. (Attorney Docket No.     P21765), filed on Jan. 7, 2005 and entitled “Centrally Articulating     Spinal Device and Method;” and -   U.S. Utility patent application Ser. No. (Attorney Docket No.     P21751), filed on Jan. 7, 2005 and entitled “Posterior Spinal Device     and Method.”

TECHNICAL FIELD

Embodiments of the invention relate generally to devices and methods for accomplishing spinal surgery, and more particularly in some embodiments, to spinal arthroplasty devices capable of being placed posteriorally into the vertebral disc space. Various implementations of the invention are envisioned, including use in total spine arthroplasty replacing, via a posterior approach, both the disc and facet functions of a natural spinal joint.

BACKGROUND

As is known the art, in the human anatomy, the spine is a generally flexible column that can take tensile and compressive loads, allows bending motion and provides a place of attachment for ribs, muscles and ligaments. Generally, the spine is divided into three sections: the cervical, the thoracic and the lumbar spine. FIG. 1 illustrates schematically the lumbar spinal 1 and the sacrum regions 3 of a healthy, human spinal column. The sections of the spine are made up of individual bones called vertebrae and the vertebrae are separated by intervertebral discs which are situated therebetween.

FIG. 2 illustrates a portion of the right side of a lumbar spinal region with a healthy intervertebral disc 5 disposed between two adjacent vertebrae 7, 9. In any given joint, the top vertebra may be referred to as the superior vertebra and the bottom one as the inferior vertebra. Each vertebra comprises a generally cylindrical body 7 a, 9 a, which is the primary area of weight bearing, and three bony processes, e.g., 7 b, 7 c, 7 d (two of which are visible in FIG. 2). As shown in FIG. 7A, in which all of the processes are visible, processes 7 b, 7 c, 7 d extend outwardly from vertebrae body 7 at circumferentially spaced locations. The processes, among other functions, provide areas for muscle and ligament attachment. Neighboring vertebrae may move relative to each other via facet components 7 e (FIG. 2), which extend from the cylindrical body of the vertebrae and are adapted to slide one over the other during bending to guide movement of the spine. There are two facet joints, each defined by upper and lower facet components, associated with adjacent vertebra. A healthy intervertebral disc is shown in FIG. 3. As shown in FIG. 3, an intervertebral disc has 4 regions: a nucleus pulposus 11, a transition zone 13, an inner annulus fibrosis region 15 and an outer annulus fibrosis 17. Generally, the inner annulus fibrosis region 15 and the outer annulus fibrosis region 17 are made up of layers of a fibrous gristly material firmly attached to the vertebral bodies above and below it. The nucleus pulposus 11 is typically more hydrated in nature.

These intervertebral discs function as shock absorbers and as joints. They are designed to absorb the compressive and tensile loads to which the spinal column may be subjected while at the same time allowing adjacent vertebral bodies to move relative to each other a limited amount, particularly during bending (flexure) of the spine. Thus, the intervertebral discs are under constant muscular and/or gravitational pressure and generally are the first parts of the lumbar spine to show signs of “wear and tear”.

Facet joint degeneration is also common because the facet joints are in almost constant motion with the spine. In fact, facet joint degeneration and disc degeneration frequently occur together. Generally, although one may be the primary problem while the other is a secondary problem resulting from the altered mechanics of the spine, by the time surgical options are considered, both facet joint degeneration and disc degeneration typically have occurred. For example, the altered mechanics of the facet joints and/or intervertebral disc may cause spinal stenosis, degenerative spondylolisthesis, and degenerative scoliosis.

One surgical procedure for treating these conditions is spinal arthrodesis (i.e., spine fusion), which has been performed both anteriorally and/or posteriorally. The posterior procedures include in-situ fusion, posterior lateral instrumented fusion, transforaminal lumbar interbody fusion (“TLIF”) and posterior lumbar interbody fusion (“PLIF”). Solidly fusing a spinal segment to eliminate any motion at that level may alleviate the immediate symptoms, but for some patients maintaining motion may be advantageous. It is also known to surgically replace a degenerative disc or facet joint with an artificial disc or an artificial facet joint, respectively. However, none of the known devices or methods provide the advantages of the embodiments of the present disclosure.

Accordingly, the foregoing shows there is a need for an improved spinal arthroplasty that avoids the drawbacks and disadvantages of the known implants and surgical techniques.

SUMMARY

A first embodiment of this disclosure provides an artificial spinal joint for creating at least a portion of a coupling between a superior vertebra and an inferior vertebra. The artificial spinal joint may include an anterior joint replacement component and a bridge coupled to the anterior joint replacement that extends posteriorly from the anterior joint replacement. Additionally, a posterior joint replacement component may be coupled to the bridge as part of the artificial spinal joint.

In another embodiment, the artificial spinal joint includes an anterior joint replacement component with a left upper member, a left lower member that articulates with the left upper member, a right upper member, and a right lower member that articulates with the right upper member. In this embodiment, the left lower member and the right lower member are each configured to connect to the other.

In still another embodiment, this disclosure describes a method of implanting an artificial spinal joint. The method includes making an incision in a patient's back, removing at least a portion of spinal disc material from a spinal disc, the removing comprising accessing the disc material through the incision, and inserting an implant through the incision. In this embodiment, an implant that replaces at least a portion of a disc and a facet joint is inserted.

The embodiments disclosed may be useful for degenerative changes of the lumbar spine, post-traumatic, discogenic, facet pain or spondylolisthesis, and/or to maintain motion in multiple levels of the lumbar spine.

Additional and alternative features, advantages, uses and embodiments are set forth in or will be apparent from the following description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation schematic view of the lumbar spinal and the sacrum regions of a healthy, human spinal column.

FIG. 2 is a detailed perspective view showing a portion of the right side of the lumbar vertebrae shown in FIG. 1 with a healthy disc disposed between two vertebrae.

FIG. 3 is a top perspective view of the intervertebral disc shown in FIG. 2 illustrating the major portions of the disc.

FIG. 4 is a side exploded elevation view of a portion of a lumbar spine showing a first embodiment of an artificial intervertebral joint constructed according to the principles of the disclosure.

FIG. 5 is an anterior elevation view of a portion of a lumbar spine showing the superior, disc and inferior portions of the left and right halves of an assembled artificial intervertebral joint constructed according to the first embodiment of the disclosure.

FIG. 6 is a side elevation view of the right half of the artificial intervertebral joint shown in FIG. 5.

FIG. 7A is a transverse, bottom-up-view of a portion of a lumbar spine showing the superior portion of the artificial intervertebral joint illustrated in FIG. 4.

FIG. 7B is a transverse, top-down-view of a portion of a lumbar spine showing the inferior portion of the artificial intervertebral joint illustrated in FIG. 4.

FIG. 8 is a transverse, bottom-up-view of a portion of a lumbar spine showing a second embodiment of a superior portion of an artificial intervertebral joint in which pedicle screws are used to assist in implantation.

FIG. 9 is a transverse, top-down-view of a portion of a lumbar spine showing a second embodiment of an inferior portion of an artificial intervertebral joint in which pedicle screws are used to assist in implantation.

FIG. 10 is a lateral view of a portion of a lumbar spine showing the superior portion of the artificial intervertebral joint shown in FIG. 8 with one of the pedicle screws being visible.

FIG. 11 is a lateral view of a portion of a lumbar spine showing the inferior and integrated disc portions of an artificial integral intervertebral joint shown in FIG. 9 with one of the pedicle screws being visible.

FIG. 12 is a posterior view of a portion of a lumbar spine showing the superior portion of the artificial intervertebral joint shown in FIG. 8 with two pedicle screws being visible.

FIG. 13 is a posterior view of a portion of a lumbar spine showing the inferior portion of the artificial intervertebral joint shown in FIG. 9 with two pedicle screws being visible.

FIG. 14 is a side elevation view of a portion of a lumbar spine showing the second embodiment with pedicle screws in an assembled position.

FIG. 15 is a posterior view of a portion of a lumbar spine showing a third embodiment of the inferior, disc and superior portions of an artificial intervertebral joint in which tension bands are used.

FIG. 16 is a side elevation view of a portion of a lumbar spine showing the third embodiment in which tension bands are used in an assembled position.

FIG. 17 is a transverse, bottom-up-view of a portion of a lumbar spine showing the superior portion of a fourth embodiment of an artificial intervertebral joint constructed according to the principles of the disclosure in which the facet joints are not replaced.

FIG. 18 is a transverse, top-down-view of a portion of a lumbar spine showing the inferior portion of the fourth embodiment of an artificial intervertebral joint.

DESCRIPTION

The drawings illustrate various embodiments of an artificial intervertebral joint for replacing an intervertebral disc or the combination of an intervertebral disc and at least one corresponding facet joint. Various embodiments of the artificial intervertebral joint according to the principles of the disclosure may be used for treating any of the problems that lend themselves to joint replacement including particularly, for example, degenerative changes of the lumbar spine, post-traumatic, discogenic, facet pain or spondylolisthesis and/or to maintain motion in multiple levels of the lumbar spine.

FIGS. 4-7 illustrate a first exemplary embodiment of an artificial intervertebral joint. As illustrated in FIGS. 4 and 5, each joint is composed of two arthroplasty halves, each of which has a spacer or disc 19 and a retaining portion 21. The retaining portion 21 includes a first retaining portion 21 a and a second retaining portion 21 b. In the example illustrated in FIG. 4, the first retaining portion 21 a is superior to (above) the second retaining portion 21 b and the disc 19 is situated therebetween. Although the artificial intervertebral joint according to this exemplary embodiment has two halves for each of the first retaining portion and the second retaining portion, it should be understood that alternative embodiments may be implemented such that the artificial intervertebral joint has a single first retaining member, a single second retaining member and a single spacer. It should also be understood that alternative embodiments may also be carried out with arthroplasties having a first retaining portion, a second retaining portion, and/or a disc which each consist of unequal sized halves or more than two components.

Further, as illustrated in FIG. 4, the first retaining portion 21 a and the second retaining portion 21 b are situated between two adjacent vertebrae. More particularly, the first retaining portion may be situated along an inferior surface of the upper of the two adjacent vertebrae and the second retaining portion may be situated above a superior surface of the lower of the two adjacent vertebrae. However, it should be understood by one of ordinary skill in the art that the first retaining portion and second retaining portion are not limited to such an arrangement, and may be oriented in different positions and/or shaped differently than what is illustrated herein.

The surfaces of the retaining portions 21 a, 21 b of the arthroplasty that contact the remaining end plates of the vertebrae may be coated with a beaded material or plasma sprayed to promote bony ingrowth and a firm connection therebetween. In particular, the surface to promote bone ingrowth may be a cobalt chromium molybdenum alloy with a titanium/calcium/phosphate double coating, a mesh surface, or any other effective surface finish. Alternatively or in combination, an adhesive or cement such as polymethylmethacrylate (PMMA) may be used to fix all or a portion of the implants to one or both of the endplates.

As discussed in more detail below, a significant portion of the outer annulus region 17 (see, e.g., FIGS. 4, 7B), in some embodiments about 300 degrees, may be retained on the inferior portion of the end plate, which acts as a stop retaining the lower retaining portions in place until bone ingrowth occurs to firmly attach the retaining portions to their respective vertebrae (FIG. 4 only shows a portion of the outer annulus 17 that is retained). In contrast, in conventional anterior arthroplasty about 270 degrees of the outer annulus region 17 typically is removed. In addition, pedicle screws may also be used for immediate fixation as described in more detail in connection with other embodiments discussed below.

In the various embodiments of this disclosure, the first retaining portion 21 a and the second retaining portion 21 b are structured so as to retain the disc 19 therebetween. For example, in the case of a disc 19 with two convex surfaces 19 a, each of the first retaining portion 21 a and the second retaining portion 21 b may have a concave surface 21 c which defines a space within which the disc 19 may be retained. For example, in the exemplary embodiment shown in FIG. 4, the upper convex surface 19 a of the disc 19 fits within the concavity defined by the concave surface 21 c of the first retaining portion 21 a and the lower convex surface 19 b of the disc 19 fits within the concavity defined by the concave surface 21 c of the second retaining portion 21 b.

FIG. 5 illustrates an anterior view of an exemplary assembled artificial intervertebral joint with both arthroplasty halves in place, and FIG. 6 shows a side view of the assembled artificial intervertebral joint shown in FIG. 5. As illustrated in FIGS. 5 and 6, the disc 19 is retained between the first retaining portion 21 a and the second retaining portion 21 b. It should be understood that although the disc 19 may be held between the first retaining portion 21 a and the second retaining portion 21 b, the disc 19 is free to slidably move within the space defined by the corresponding surfaces 21 a of the first retaining portion 21 a and the second retaining portion 21 b. In this manner, limited movement between the adjacent vertebrae is provided.

In the exemplary embodiment illustrated in FIGS. 4, 5 and 6, the disc 19 is a separate component which is inserted between the first retaining portion 21 a and the second retaining portion 21 b. However, as discussed below, it should be understood that the spacer or disc 19 may be integrally formed with or integrated into in one or both of the first retaining portion 21 a and the second retaining portion 21 b.

In the exemplary embodiment of the disclosure, as illustrated best in FIGS. 4, 6, 7A and 7B, each of the retaining portions of the artificial intervertebral joint includes a first artificial facet component 23 a and a second artificial facet component 23 b. As shown in FIGS. 7A and 7B, the first artificial facet component 23 a has a face 25 a and the corresponding second artificial facet component 23 b has a face 25 b configured such that the face 25 a matingly fits with the face 25 b to stabilize adjacent vertebrae while preserving and guiding the mobility of each vertebrae with respect to the other vertebrae. Each set of the upper and lower retaining portions 21 a, 21 b may have a pair of facet components 23 a, 23 b, which together define a facet joint. For a total joint replacement with facets according to this embodiment, the left and right arthroplasties would define two adjacent facet joints when viewed from the posterior.

Regardless of whether artificial facet joints are provided, the respective upper and lower retaining portions associated with the left and right halves of the arthroplasty may be completely independent from the other. That is, as shown in FIG. 7A, for example, the first retaining portions 21 a associated with each half are not in direct contact with each other. The same is true with respect to the second retaining portions 21 b shown in FIG. 7B. However, it should be understood by one of ordinary skill in the art that, even in the embodiment of the disclosure which includes artificial facet joints, at least a portion of the first retaining portions 21 a of each half and/or at least a portion of the second retaining portions 21 b of each half may directly contact and/or be connected to each other as described in more detail in connection with the discussion of FIGS. 17-18.

Further, in the various embodiments of the disclosure, the disc 19, the first retaining portion 21 a and the second retaining portion 21 b may be made of any appropriate material which will facilitate a connection that transmits compressive and tensile forces while providing for the aforementioned slidable motion in a generally transverse direction between each of the adjacent surfaces. For example, in the first embodiment, the first retaining portion 21 a and the second retaining portion 21 b may be typically made from any metal or metal alloy suitable for surgical implants such as stainless steel, titanium, and cobalt chromium, or composite materials such as carbon fiber, or a plastic material such as polyetheretherketone (PEEK) or any other suitable materials. The disc may be made from plastic such as high molecular weight polyethylene or PEEK, or from ceramics, metal, and natural or synthetic fibers such as, but not limited to, carbon fiber, rubber, or other suitable materials. Generally, to help maintain the sliding characteristic of the surfaces, the surfaces may be polished and/or coated to provide smooth surfaces. For example, if the surfaces are made of metal, the metal surfaces may be polished metal.

FIGS. 8-14 illustrate a second embodiment of an artificial intervertebral joint. Only features that differ from the first embodiment are discussed in detail herein. In the second exemplary embodiment, securing components, such as, for example, pedicle screws 27 are provided to provide a more secure and immediate connection between each of the first retaining portion 21 a and/or the second retaining portion 21 b to the corresponding vertebra. In addition, this embodiment illustrates a disc 19 which is integrated with one of the retaining portions, here lower retaining portion 21 b. Disc 19 may be integrally formed from the same material as its retaining portion, but also may be separately formed from similar or dissimilar materials and permanently connected thereto to form an integral unit. In this embodiment, the disc 19 and the retaining portions may be all formed from metal.

FIGS. 15 and 16 illustrate a third embodiment of an artificial intervertebral joint. In the third exemplary embodiment, additional securing components, such as, for example, tension bands 31 are provided to supplement or replace the function of posterior ligaments that limit the mobility between adjacent vertebrae by securing the first retaining portion 21 a to the second retaining portion 21 b. As shown in FIGS. 15-16, posterior tension bands 31 may be provided by wrapping them around the corresponding pedicle screws 27 or other convenient attachment points.

FIGS. 17 and 18 illustrate a fourth embodiment of an artificial intervertebral joint. In the exemplary embodiment illustrated in FIGS. 17 and 18, the artificial intervertebral joint may have all of the features discussed above except for artificial facet components. In this embodiment, the natural facet joints remain. The ligamentous tension band may also be left intact in some embodiments. In addition, this embodiment includes a specific example of an anterior midline connection between respective upper and lower retaining portions, which assists in maintaining the placement of the first retaining portion 21 a and the second retaining portion 21 b.

FIGS. 17 and 18 illustrate that it is possible to provide a first retaining portion 21 a with a lock and key type pattern which is complemented by the corresponding mating portion provided on the second retaining portion 21 b. More particularly, one half of the first retaining portion 21 a has an outer boundary with a U-shaped portion 35 a while the other half of the corresponding first retaining portion 21 a has an outer boundary with a protruding portion 35 b, which fits into the U-shaped portion 35 a. As a result, each half of the first retaining portion 21 a, 21 b may be maintained in a predetermined position. However, the upper or lower retaining portions may fit together and/or be connected in the interbody space, e.g., near their midline anterior portions, in any manner that facilitates implantation and/or assists in providing and/or retaining the joint in a generally stable, symmetrical configuration. It may be even more important to provide such connection between the lower retaining portions due to the inward forces provided by annulus 17 remaining on the inferior end plate as shown in FIG. 18. A midline connection between the respective lower retaining portions will resist the force of the outer annulus tending to cause migration of the retaining portions toward the midline 37.

As shown in the various exemplary embodiments, other than the portions of the first and/or second retaining portions which may fit together like a lock and key to maintain the placement of the portions relative to each other, each half of the artificial intervertebral joint may be generally symmetrical about the midline 37 of the vertebrae.

Again, these exemplary embodiments are merely illustrative and are not meant to be an exhaustive list of all possible designs, implementations, modifications, and uses of the invention. Moreover, features described in connection with one embodiment of the disclosure may be used in conjunction with other embodiments, even if not explicitly stated above.

While it should be readily apparent to a skilled artisan from the discussion above, a brief description of a suitable surgical procedure that may be used to implant the artificial joint is provided below. Generally, as discussed above, the artificial intervertebral joint may be implanted into a body using a posterior transforaminal approach similar to the known TLIF or PLIF procedures. According to this approach, an incision, such as a midline incision, may be made in the patient's back and some or all of the affected disc and surrounding tissue may be removed via the foramina. Depending on whether any of the facet joints are being replaced, the natural facet joints may be trimmed to make room for the artificial facet joints. Then, the halves of the artificial intervertebral joint may be inserted piecewise through the left and right transforaminal openings, respectively. That is, the pieces of the artificial intervertebral joint including the upper and lower retaining portions, with or without facet components, and the artificial disc, if provided separately, fit through the foramina and are placed in the appropriate intervertebral space. The pieces of the artificial joint may be completely separated or two or more of them may be tied or packaged together prior to insertion through the foramina by cloth or other materials known in the art. In cases where at least a portion of the outer annulus of the natural disc can be retained, the lower retaining portions of each side of the artificial intervertebral joint are inserted such that they abut a corresponding portion of the annulus. If a midline anterior connection is provided, the left and right halves of the retaining members are fitted together and held in place by the outer annulus. As such, the remaining portion of the annulus may be in substantially the same place as it was prior to the procedure.

Further, in the cases where the annulus of the natural disc must be removed completely or this is insufficient annulus remaining, it is possible, for example, to use the embodiment of the disclosure where the pedicle screws are implemented so as to be assured that the pieces of the artificial intervertebral joint remain in place. It should be understood by one of ordinary skill in the art that the artificial joint could be implanted via an anterior approach or a combined anterior and posterior approach, although the advantages of a posterior procedure would be limited. For example, some of the pieces of the artificial intervertebral joint may be inserted from an anterior approach and others posteriorally. The anterior and posteriorally placed portions could be fitted together similar to the embodiment shown in FIGS. 17 and 18.

Although only a few exemplary embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this disclosure. Accordingly, all such modifications and alternative are intended to be included within the scope of the invention as defined in the following claims. Those skilled in the art should also realize that such modifications and equivalent constructions or methods do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. It is understood that all spatial references, such as “horizontal,” “vertical,” “top,” “upper,” “lower,” “bottom,” “left,” and “right,” are for illustrative purposes only and can be varied within the scope of the disclosure. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. 

1-79. (canceled)
 80. A system for implantation in an intervertebral space defined between first and second vertebrae to permit movement of the first vertebra relative to the second vertebra, the system comprising: a first arthroplasty section including a first upper contact portion sized to engage an endplate of the first vertebra, a first lower contact portion sized to engage an endplate of the second vertebra, an outer boundary with a first mating portion, and a first pair of articulation surfaces sized and shaped for slidable engagement; and a second arthroplasty section including a second upper contact portion sized to engage an endplate of the first vertebra, a second lower contact portion sized to engage an endplate of the second vertebra, an outer boundary with a second mating portion, and a second pair of articulation surfaces sized and shaped for slidable engagement; wherein the first mating portion is shaped to receive the second mating portion to connect the first and second arthroplasty sections within the intervertebral space.
 81. The system of claim 80 wherein the second mating portion includes a protrusion.
 82. The system of claim 81 wherein the protrusion is integrally formed with the second lower contact portion.
 83. The system of claim 81 wherein the protrusion is integrally formed with the second upper contact portion.
 84. The system of claim 80 wherein the outer boundaries of the first and second arthroplasty sections are in direct contact when the second mating portion is received within the first mating portion.
 85. The system of claim 80 wherein the first and second mating portions are sized to connect along an anterior-posterior midline axis extending through the intervertebral space.
 86. The system of claim 85 wherein the first pair of articulation surfaces are sized and shaped for slidable engagement on an opposite side of the anterior-posterior midline axis from the second pair of articulation surfaces.
 87. The system of claim 80 further comprising a fixation portion sized to extend posteriorly outside of the intervertebral space to receive at least one fastener.
 88. The system of claim 87 wherein the at least one fastener is a threaded bone fastener.
 89. The system of claim 88 wherein a tension band is attached to the at least one fastener.
 90. A system for placement in an intervertebral disc space between two vertebrae, the system comprising a pair of arthroplasty devices, each arthroplasty device including an upper portion slidably engaged with a lower portion and a mating portion shaped to interlock with the other arthroplasty device at an anterior-posterior midline through the intervertebral disc space, to resist migration of the arthroplasty devices with respect to each other.
 91. The system of claim 90 wherein the mating portions are formed along outer boundaries of the lower portions of each arthroplasty device.
 92. The system of claim 91 wherein the mating portions are formed along outer boundaries of the upper portions of each arthroplasty device.
 93. The system of claim 90 wherein one of the mating portions has a projection and the other mating portion has a U-shaped formation sized to interlock with the projection.
 94. A method of implanting a spinal arthroplasty system comprising: accessing an intervertebral disc space through a posterior incision; inserting a first arthroplasty portion into an area of the intervertebral disc space to one lateral side of an anterior-posterior midline axis defined through the intervertebral disc space, wherein the first arthroplasty portion includes a mating pair of slidable surfaces; inserting a second arthroplasty portion into an area of the intervertebral disc space to the other lateral side of the anterior-posterior midline axis, wherein the second arthroplasty portion includes a mating pair of slidable surfaces; and connecting the first arthroplasty portion to the second arthroplasty portion within the intervertebral disc space.
 95. The method of claim 94 wherein the step of connecting includes inserting a projection of the first arthroplasty portion into a mating recess of the second arthroplasty portion.
 96. The method of claim 95 wherein the projection is formed in an outer boundary of the first arthroplasty portion.
 97. The method of claim 94 wherein the first and second arthroplasty portions each include a lower portion comprising a bone engagement surface and one of the pair of slidable surfaces and wherein the step of connecting includes connecting the lower portions of the arthroplasty portions.
 98. The method of claim 94 wherein the arthroplasty portions are connected at the anterior-posterior midline axis.
 99. The method of claim 94 wherein the intervertebral disc space has an anterior area and a posterior area and the step of connecting includes interlocking the first and second arthroplasty portions in the anterior area of the intervertebral disc space. 